1. Field of the Invention
The present invention relates to the field of acoustic wave processing. Particular embodiments of the invention relate to methods and devices that modify the characteristics of acoustic waves, such as variable bandwidth acoustic wave filters and acoustic wave delay devices.
2. Related Art
The demand for products based on solid state technology in the consumer, industrial and military markets continues to grow at a rapid rate. For example, in the consumer telecommunications market, some forecasts estimate that mobile phone sales will reach an astounding 750 million to 1 billion units by 2003. Commensurate with this demand is a demand for the components inside these products, including signal processing components relying on acoustic wave phenomena. Acoustic wave devices, such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices, are used frequently in products such as televisions, cell phones and pagers.
One application for acoustic wave devices is in electronic signal filtering. Electronic filters are ubiquitous and are available in a variety of technologies. For example, in cellular telephones, filters may be implemented using analog technology, as in charge coupled devices (CCDs), digital technology, as in finite impulse response (FIR) filters, or acoustic wave technology, all of which are well-known in the art. Analog and digital filters are versatile in the sense that the frequency response of these filters can be varied. These filters, however, suffer from an inability to directly address high frequency applications, such as, for example, applications in the vast majority of wireless communications markets, either because of fundamental component frequency limitations or an adverse power-speed relationship. This is troublesome for many designers, especially those designing in the wireless electronics area, where bandwidth and frequency of operation continue to increase and power is at a premium. In some applications, the power consumption of analog and digital filters is too great for these filters to be viable as design solutions. Filters based on acoustic wave technology, however, which consume very little power in relation to analog and digital filters, offer a viable solution to this problem.
However, acoustic wave devices have not been without their problems. Historically, acoustic wave devices have been limited in flexibility. For example, SAW filters are typically used for fixed filter applications. Once fabricated, a SAW filter is fixed with respect to bandwidth and center frequency. If a different frequency response is desired, a new SAW filter must be substituted.
There have been attempts to address this problem. In U.S. Pat. No. 3,446,974, issued to Seiwatz, a solid state acoustic signal translating device, such as a surface acoustic wave filter, is described in which a photoconductive film contacts the surface electrodes of the device transducer. An optical system illuminates selected portions of the photoconductive film to establish various connections between electrodes, thereby varying the frequency response of the device, which is dependent upon the particular electrodes activated. This approach to vary the frequency response of the device has several disadvantages: there is substantial loss due to the resistive drop across the electrodes; the shape of the filter changes dramatically as different electrodes are selected; and, electrode selection is not arbitrary due to the pre-defined electrode pattern.
A purported improvement on the Seiwatz device is described in U.S. Pat. No. 3,621,482, issued to Adler. The Adler device is essentially a plurality of SAW filters selectively coupled together via a photoconductive material. Although the Adler device may improve upon the resistive loss problem of the Seiwatz device, the number of filter structures needed for the Adler device grows in proportion to the number of frequency selections to be made. This may make the Adler device too costly, too cumbersome or otherwise unusable in many instances.
Another approach is described in U.S. Pat. No. 4,099,206, issued to Desbois et al. The Desbois device consists of a substrate of piezoelectric material upon which is deposited an interdigital network and electro-acoustic transducers. The interdigital network is covered by, but electrically isolated from, a photoconductive medium. Illumination of the photoconductive material affects the conductivity of the network, thereby influencing the surface acoustic wave generated and received buy the transducers. This device also suffers from resistive losses, and its tunability is limited, as in the Seiwatz device, by the predefined geometry of the network.
Thus, there is a need to provide a method and apparatus for varying the frequency characteristics of acoustic waves, without the resistive losses of previous devices and with tunability that does not rely on a predefined control structure.
Embodiments of the present invention are directed to methods and apparatuses that address this need. In one embodiment, a method for modifying the characteristics of an acoustic wave comprises the steps of providing a medium for acoustic wave propagation, generating an acoustic wave, propagating the acoustic wave using the medium, and illuminating the medium during the propagation of the acoustic wave. The medium may be a piezoelectric substrate. A transducer may be formed on the piezoelectric substrate, and the transducer may generate the acoustic wave. Further, the medium may be illuminated using a laser diode or a light-emitting diode. The method may also comprise the step of varying the intensity of the light generated by the light-emitting diode or laser diode. The intensity of the light may be varied by a controller or a light modulator. The method may also comprise the step of reading a selected frequency component of the acoustic wave.
Another embodiment of the present invention is directed toward a method for modifying the characteristics of an acoustic wave comprising the steps of generating an acoustic wave in a medium and varying a velocity of the acoustic wave. This medium may also be a piezoelectric substrate. The velocity of the acoustic wave may be varied by illuminating the medium. Further, a transducer may be formed on the piezoelectric substrate and the acoustic wave may be generated by the transducer. In addition, the method of this embodiment may also comprise the step of reading a selected frequency component of the acoustic wave.
Another embodiment of the present invention is directed toward an apparatus for varying the characteristics of an acoustic wave comprising a medium for acoustic wave propagation, a transducer formed on the medium, and a light source illuminating the medium. The medium may be a piezoelectric substrate and the acoustic wave may be generated by the transducer. The light source may be a laser diode or a light-emitting diode and the intensity of the light generated by either light source may be varied. To vary the intensity of the light, a controller or a light modulator may be used. In the apparatus, a selected frequency component of the acoustic wave may be read from the transducer.
Another embodiment of the present invention is directed toward a method for making an acoustic wave device comprising the steps of providing a medium for acoustic wave propagation, forming a transducer on the medium and providing a light source for illuminating the medium. The medium may be a piezoelectric substrate. The light source may be a laser diode or a light-emitting diode. The method may also comprise the step of providing means for varying an intensity of a light generated by the light-emitting diode. The means for varying the intensity of the light may comprise a controller or a light modulator.
Another embodiment of the present invention is directed toward an apparatus for varying the characteristics of an acoustic wave comprising a medium for acoustic wave propagation and a transducer formed on the medium, wherein a light source is used to illuminate the medium. The medium may be a piezoelectric substrate. The transducer may generate an acoustic wave. The light source may be a laser diode or a light-emitting diode. In this embodiment, an intensity of a light generated by the laser diode or light-emitting diode is varied, preferably by a controller or a light modulator. A selected frequency component of the acoustic wave is read from the transducer.
Another embodiment of the present invention is directed toward a method for modifying the characteristics of an acoustic wave comprising the steps of providing a medium for acoustic wave propagation, generating an acoustic wave, propagating the acoustic wave using the medium, and inducing a charge grating in the medium during the propagation of the acoustic wave. The medium may be a piezoelectric substrate and a transducer may be formed on the piezoelectric substrate.
Another embodiment of the present invention is directed toward a method for making an acoustic wave device comprising the steps of providing a medium for acoustic wave propagation, forming a transducer on the medium, and providing a light source for inducing a charge grating in the medium. The medium may be a piezoelectric substrate. The light source may be a laser diode or a light-emitting diode. The method may also comprise the step of providing means for varying an intensity of a light generated by the light-emitting diode, wherein the means for varying the intensity of the light comprises a controller or a light modulator.